There are numerous examples in the prior art of reducing the torque exerted by a conveyed fluid on the valve plate of a butterfly valve, where the torque reduction results from a reduction in the effective area of the valve plate.
In some cases, as illustrated by U.S. Pat. No. 2,796,082 Green, the area reduction is provided in the form of a pilot valve used to facilitate initial opening of the valve plate. This does not significantly reduce torque over a broad range of rotational positions of the valve plate.
In other cases, as illustrated by U.S. Pat. No. 3,971,414 Illing, a reduction in the effective area of the valve plate is used to effect a significant overall reduction in the torque applied to the valve plate by the conveyed fluid. However, structures such as that disclosed by Illing present difficulty in applications which demand sealing at the closed rotational position of the valve plate. More importantly, the reduction in effective area, as a function of the rotational position of the valve plate, is fixed by the design of the valve structure described by Illing. This limits the effectiveness of the valve in applications characterized by variable pressure ratios, since the optimal functional relationship depends on the pressure ratio (upstream/downstream) of the conveyed fluid.